Well unfortunately I don't live in Europe and no one talks about it here where I live so I'm not sure why you automatically expect me to know something I've never heard of
I've heard that at high tide the path connecting it to land gets engulfed by water, making it an island. Maybe it happened for extra long or something.
So this happened, the rides were about 4 meters higher than usual and the Mont saint Michel, which is connected to france by a wooden bridge was completely surrounded by water, this is also cause the sand that actually doesnt belong around the island is being washed away by a new " flushing dam"
It''s amazing that the island was once a hill in the midle of a forrest, but in the medieval times they cut down the trees and the sea took over.
It's not a definition of what happens when you just say how often it happens..
It''s amazing that the island was once a hill in the midle of a forrest, but in the medieval times they cut down the trees and the sea took over.